1. Field of the Invention
The present invention relates to a wire bonding method in which a wire that passes through a capillary is connected to a first bonding point and a second bonding point, and more particularly, to a wire bonding method which forms a low wire loop in low-step-height devices in which the step height between the first and second bonding points is small.
2. Prior Art
Various types of wire bonding methods have been proposed. The most common method is shown in FIGS. 8(a) and 8(b).
As seen from FIG. 8(a), a capillary 2 is lowered with a damper (not shown), which is provided above the capillary 2 so as to hold a wire 1, in an open state; and a ball formed on the tip end of the wire protruding from the tip end of the capillary 2 is bonded to a first bonding point A.
Japanese Patent Application Laid-Open No. H10-125715 discloses a method for forming a ball at the end of a bonding wire. In this prior art, a charging electrode is provided so as to be positioned on one side of a bonding wire, and a magnet is used so that the discharge spark from the electrode is applied to the wire from underneath the wire.
After the ball is bonded to the first bonding point A, the capillary 2 is raised to point B, delivering the wire 1 by an amount that corresponds to the wire length required to form a wire loop. Next, the clamper is closed, and the capillary 2 is moved to point D which is straight above the second bonding point C by performing a circular-arc motion as indicated by a curved arrow.
Then, as shown in FIG. 8(b), the capillary 2 is lowered, and the wire 1 is bonded to the second bonding point C. Afterward, the damper is opened, and the capillary 2 and the damper are both raised. During this raising movement, the damper is closed so as to cut the wire 1 from its attachment root at the second bonding point C. The wire bonding of one wire is thus completed.
A wire bonding method of the type described above is disclosed in Japanese Patent Application Publication (Kokoku) No. HI-26531.
The conventional wire bonding method as described has problems when bonding is performed for a low-step-height device in which the step height between the first bonding point A and the second bonding point B is as small as, for instance, 50 microns or less. For instance, as shown in FIG. 8(a), when the capillary 2 is moved to a point slightly above the second bonding point C, an excess portion of the wire 1 is suspended from the lower end of the capillary 2, resulting in a wire shape 11 in which a bowed portion 10 is formed. Because of this bowed portion 10, the wire length that is delivered from the capillary 2 is in fact longer than the ideal wire length of the wire shape 12 that is indicated by the two-dot chain line in FIG. 8(a). As a result of this excess wire, the wire 1 undergoes a rebound due to plastic deformation when the wire 1 is bonded to the second bonding point C; and a wire loop shape 13 with an upward bow such as that shown in FIG. 8(b) is formed.
In a low wire loop in which the apex of the loop is supposed to be the portion 14 above the first bonding point A, the wire loop shape 13 which has such a bow increases the overall height of the wire loop, and it also makes it difficult to control the height of the wire loop in the looping operation. Thus, an unstable wire loop shape is formed. The two-dot chain line shown in FIG. 8(b) shows an ideal wire loop shape 19.